I had some interesting results with the 'SA Watch' and Google Earth software used in conjuction with a Garmin eTrex Camo GPS SW ver 3.7 (Discontinued but with the latest firmware).

The Garmin eTrex is the older standard chipset and not the high sensitivity H or HC models etc.

WAAS (EGNOS in Europe) was enabled and the NMEA 0183 output (4800,8,N,1) was selected. The eTrex was situated on a south facing window sill and could observe only half the sky with some large trees obscuring the southerly view. So not the best sky view. The Garmin eTrex was still able to view 6-7 SVs (many of them to the North which was obscured so considerable multipath must have been at play). The EGNOS PRN was detected and DGPS positional computations were being carried out by the eTrex unit. (typically the error estimation by the eTrex was typically around 3-6 metres with SA view showing a CEP @ 95% around the same value. Not bad considering the poor sky view)

The Garmin eTrex supports the NMEA 0183 sentences GPGGA, GPGLL, GPGSA, GPGSV, GPRMB, GPRMC, GPRMT, GPVGT, GPWLP, GPBOD

None of these message sentences in the NMEA format indicate the Datum as indicated in the NMEA sentence reference i.e GPDTM

http://www.tronico.fi/OH6NT/docs/NMEA0183.pdf

This is where there appears to be a problem with the eTrex unit. When ever the Datum is changed a significant error may occur if the eTrex is communicating its position to another device if the other electronic mapping/recording/navigational device isn't aware of a datum change by the eTrex GPS.

Initially the eTrex was set to the OSGB36 datum for UK Ordnance Survey - the eTrex generated a stable 8 digit grid reference accurate to 10m, which was spot on (excellent considering this is about as accurate as any 1:25,000 scale mapping work would ever require and considering the poor sky view and multipath error that must have been occurring)

What wasn't so great was when I went to use Google Earth (which I assume uses the WGS84 datum) with the eTrex using the OSGB36 datum. The position was consistently out by 90m to the east. At first I was at a loss as to what was happening, then realised the difference must be down to the difference in the Google Earth WGS84 datum and the OSGB36 datum. As soon as I reconfigured the Datum on the eTrex to the WGS84 datum, the Google Earth pinpointed the window on my house on the aerial photo.

If Garmin aren't going to include the $GPDTM output message when the eTrex talks to another device application such as external electronic mapping applications then I would have expected the output NMEA sentences to have remained referenced to the WGS84 datum. So if using your eTrex in conjuction with Google Earth on your 3G enabled netbook, you need to ensure that the eTrex has the WGS84 datum selected. As always make sure that both your map and GPS are using the same Datum, whether the map is paper or electronic. This is probably more important than losing a couple of SVs in the next 5 years from the constellation where general navigation work is required. Survey work, now that might be a whole different ball game.


Edit - An update on the eTrex Camo accuracy





The blue portion in the Observed GPS horizon is the actual sky view and the orange portion would have been subject to multipath error. As you can see there wasn't a good view of the sky and the accuracy has suffered as a result. Most of the observed measurements 72% had an HDOP > 2.0

Actual accuracy is around 20 metres CEP @ 95% rather than the earlier assumption of 3-6 metres. Again I still would actually consider this to be an excellent result considering the poor HDOP due to the limited sky view and the inherent multipath errors.